The present invention relates to case hardening iron-based articles substantially without formation of carbides.
Case hardening is a widely used industrial process for enhancing the surface hardness of metal articles. In a typical commercial process, the workpiece is contacted with a carburizing gas at elevated temperature whereby carbon atoms diffuse into the article surface. Hardening occurs through the formation of carbide precipitates, generally referred to simply as xe2x80x9ccarbidesxe2x80x9d. Gas carburization is normally accomplished at 1700xc2x0 F. (950xc2x0 C.) or above, since most steels need to be heated to these temperatures to convert their phase structures to austenite, which is necessary for carbon diffusion. In general. see Stickels., xe2x80x9cGas Carburizingxe2x80x9d,pp 312 to 324, Volume 4, ASM handbook copyright 1991, ASM International.
Carbide precipitates not only enhance surface hardness, they also promote corrosion. For this reason, stainless steel is rarely case hardened by conventional gas carburization, since the xe2x80x9cstainlessxe2x80x9d quality of the steel is compromised.
In our earlier application Ser. No. 9/133,040, filed Aug. 12, 1998, now U.S. Pat. No. 6,093,310 we describe a technique for case hardening stainless steel in which the workpiece is gas carburized below 1000xc2x0 F. At these temperatures, and provided that carburization does not last too long, the workpiece will carburize with little or no formation of carbide precipitates. As a result, the workpiece surface not only becomes hardened but also the inherent corrosion resistance of the stainless steel is maintained.
See, also. U.S. Pat. No. 5,792,282, EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364).
Although low temperature gas carburization processes can achieve hardened stainless steel products with superior corrosion resistance, it is always desirable too improve such processes to achieve faster, more-economical operation.
Accordingly, it is an object of the present invention to provide a modified low temperature gas carburization process for case hardening stainless steel and other ferrous-based materials which allows faster carburization than possible in the past and thereby reduces the overall cost of such procedures.
This and other objects are accomplished by the present invention which is based on the discovery that the rate of workpiece carburization in a low temperature carburization process can be increased by adjusting the temperature of carburization and/or the concentration of the carburization specie in the carburizing gas to approach but not exceed predetermined limits which foster carbide precipitate formation.
Accordingly, the present invention provides a new process for low temperature gas carburizing a workpiece containing iron, nickel or both comprising contacting the workpiece with a carburizing gas at an elevated carburizing temperature sufficient to promote diffusion of carbon into the surfaces of the article but insufficient to promote substantial formation of carbide precipitates in the article surfaces, wherein the carburizing temperature is lowered from an initial carburizing temperature to a final carburizing temperature so as to achieve faster carburization than possible for carburization carried out at the final carburizing temperature only.
In addition, the present invention also provides a new process for low temperature gas carburizing a workpiece containing iron, nickel or both comprising contacting the workpiece with a carburizing gas at an elevated carburizing temperature sufficient to promote diffusion of carbon into the surfaces of the article but insufficient to promote substantial formation of carbide precipitates in the article surfaces, wherein the concentration of the carburizing specie in the carburizing gas is lowered from an initial concentration to a final concentration during carburization so as to achieve a harder case than possible for carburization carried out at the final concentration only and, in addition less soot generation than possible for carburization carried out at the initial concentration only.
Still further, the present invention also provides a new process for low temperature gas carburizing a stainless steel workpiece comprising activating the surfaces of the workpiece to be carburized to make these surfaces previous to carbon atoms and then contacting the workpiece with a carburizing gas at an elevated carburizing temperature sufficient to promote diffusion of carbon into the surfaces of the article but insufficient to promote substantial formation of carbide precipitates in the article surfaces, wherein after carburization is at least 10% complete as measured by the amount of carbon taken up by the workpiece surfaces but before carburization is 80% complete, carburization is interrupted and the workpiece is reactivated to enhance diffusion of carbon atoms into the workpiece surfaces.
In yet still another aspect, the present invention also provides a new process for case hardening a workpiece by gas carburization in which a workpiece electroplated with iron is contacted with a carburizing gas at an elevated carburization temperature to cause carbon to diffuse into the workpiece surfaces thereby forming a hardened case of predetermined thickness, wherein after carburization has started but before carburization is completed carburization is interrupted and the workpiece is contacted with a purging gas consisting essentially of an inert gas at a purging temperature below 600xc2x0 F. so that the case formed at the end of carburization is harder than the case that would have been formed without contact with the purging gas.